From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 80778C07542 for ; Mon, 27 May 2019 07:55:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 49BB72070D for ; Mon, 27 May 2019 07:55:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726335AbfE0Hzq (ORCPT ); Mon, 27 May 2019 03:55:46 -0400 Received: from mga07.intel.com ([134.134.136.100]:9880 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725943AbfE0Hzp (ORCPT ); Mon, 27 May 2019 03:55:45 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga105.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 27 May 2019 00:55:45 -0700 X-ExtLoop1: 1 Received: from yhuang-dev.sh.intel.com (HELO yhuang-dev) ([10.239.159.29]) by orsmga008.jf.intel.com with ESMTP; 27 May 2019 00:55:42 -0700 From: "Huang\, Ying" To: Yang Shi Cc: , , , , , , , , , , Subject: Re: [v6 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout References: <1558929166-3363-1-git-send-email-yang.shi@linux.alibaba.com> <1558929166-3363-2-git-send-email-yang.shi@linux.alibaba.com> <87ef4k8jgs.fsf@yhuang-dev.intel.com> Date: Mon, 27 May 2019 15:55:41 +0800 In-Reply-To: (Yang Shi's message of "Mon, 27 May 2019 15:40:52 +0800") Message-ID: <87a7f88h6q.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Yang Shi writes: > On 5/27/19 3:06 PM, Huang, Ying wrote: >> Yang Shi writes: >> >>> Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after >>> swapped out"), THP can be swapped out in a whole. But, nr_reclaimed >>> and some other vm counters still get inc'ed by one even though a whole >>> THP (512 pages) gets swapped out. >>> >>> This doesn't make too much sense to memory reclaim. For example, direct >>> reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP >>> could fulfill it. But, if nr_reclaimed is not increased correctly, >>> direct reclaim may just waste time to reclaim more pages, >>> SWAP_CLUSTER_MAX * 512 pages in worst case. >>> >>> And, it may cause pgsteal_{kswapd|direct} is greater than >>> pgscan_{kswapd|direct}, like the below: >>> >>> pgsteal_kswapd 122933 >>> pgsteal_direct 26600225 >>> pgscan_kswapd 174153 >>> pgscan_direct 14678312 >>> >>> nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would >>> break some page reclaim logic, e.g. >>> >>> vmpressure: this looks at the scanned/reclaimed ratio so it won't >>> change semantics as long as scanned & reclaimed are fixed in parallel. >>> >>> compaction/reclaim: compaction wants a certain number of physical pages >>> freed up before going back to compacting. >>> >>> kswapd priority raising: kswapd raises priority if we scan fewer pages >>> than the reclaim target (which itself is obviously expressed in order-0 >>> pages). As a result, kswapd can falsely raise its aggressiveness even >>> when it's making great progress. >>> >>> Other than nr_scanned and nr_reclaimed, some other counters, e.g. >>> pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed >>> too since they are user visible via cgroup, /proc/vmstat or trace >>> points, otherwise they would be underreported. >>> >>> When isolating pages from LRUs, nr_taken has been accounted in base >>> page, but nr_scanned and nr_skipped are still accounted in THP. It >>> doesn't make too much sense too since this may cause trace point >>> underreport the numbers as well. >>> >>> So accounting those counters in base page instead of accounting THP as >>> one page. >>> >>> nr_dirty, nr_unqueued_dirty, nr_congested and nr_writeback are used by >>> file cache, so they are not impacted by THP swap. >>> >>> This change may result in lower steal/scan ratio in some cases since >>> THP may get split during page reclaim, then a part of tail pages get >>> reclaimed instead of the whole 512 pages, but nr_scanned is accounted >>> by 512, particularly for direct reclaim. But, this should be not a >>> significant issue. >>> >>> Cc: "Huang, Ying" >>> Cc: Johannes Weiner >>> Cc: Michal Hocko >>> Cc: Mel Gorman >>> Cc: "Kirill A . Shutemov" >>> Cc: Hugh Dickins >>> Cc: Shakeel Butt >>> Cc: Hillf Danton >>> Signed-off-by: Yang Shi >>> --- >>> v6: Fixed the other double account issue introduced by v5 per Huang Ying >>> v5: Fixed sc->nr_scanned double accounting per Huang Ying >>> Added some comments to address the concern about premature OOM per Hillf Danton >>> v4: Fixed the comments from Johannes and Huang Ying >>> v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly >>> Switched back to use compound_order per Matthew >>> Fixed more counters per Johannes >>> v2: Added Shakeel's Reviewed-by >>> Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski >>> >>> mm/vmscan.c | 47 +++++++++++++++++++++++++++++++++++------------ >>> 1 file changed, 35 insertions(+), 12 deletions(-) >>> >>> diff --git a/mm/vmscan.c b/mm/vmscan.c >>> index b65bc50..378edff 100644 >>> --- a/mm/vmscan.c >>> +++ b/mm/vmscan.c >>> @@ -1118,6 +1118,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, >>> int may_enter_fs; >>> enum page_references references = PAGEREF_RECLAIM_CLEAN; >>> bool dirty, writeback; >>> + unsigned int nr_pages; >>> cond_resched(); >>> @@ -1129,7 +1130,10 @@ static unsigned long >>> shrink_page_list(struct list_head *page_list, >>> VM_BUG_ON_PAGE(PageActive(page), page); >>> - sc->nr_scanned++; >>> + nr_pages = 1 << compound_order(page); >>> + >>> + /* Account the number of base pages even though THP */ >>> + sc->nr_scanned += nr_pages; >>> if (unlikely(!page_evictable(page))) >>> goto activate_locked; >>> @@ -1250,7 +1254,7 @@ static unsigned long shrink_page_list(struct list_head *page_list, >>> case PAGEREF_ACTIVATE: >>> goto activate_locked; >>> case PAGEREF_KEEP: >>> - stat->nr_ref_keep++; >>> + stat->nr_ref_keep += nr_pages; >>> goto keep_locked; >>> case PAGEREF_RECLAIM: >>> case PAGEREF_RECLAIM_CLEAN: >>> @@ -1306,6 +1310,15 @@ static unsigned long shrink_page_list(struct list_head *page_list, >>> } >>> /* >>> + * THP may get split above, need minus tail pages and update >>> + * nr_pages to avoid accounting tail pages twice. >>> + */ >>> + if ((nr_pages > 1) && !PageTransHuge(page)) { >>> + sc->nr_scanned -= (nr_pages - 1); >>> + nr_pages = 1; >>> + } >> After checking the code again, it appears there's another hole in the >> code. In the following code snippet. >> >> if (!add_to_swap(page)) { >> if (!PageTransHuge(page)) >> goto activate_locked; >> /* Fallback to swap normal pages */ >> if (split_huge_page_to_list(page, >> page_list)) >> goto activate_locked; >> #ifdef CONFIG_TRANSPARENT_HUGEPAGE >> count_vm_event(THP_SWPOUT_FALLBACK); >> #endif >> if (!add_to_swap(page)) >> goto activate_locked; >> } >> >> >> If the THP is split, but the first or the second add_to_swap() fails, we >> still need to deal with sc->nr_scanned and nr_pages. >> >> How about add a new label before "activate_locked" to deal with that? > > It sounds not correct. If swapout fails it jumps to activate_locked > too, it has to be handled in if (!add_to_swap(page)). The below fix > should be good enough since only THP can reach here: > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 378edff..fff3937 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1294,8 +1294,15 @@ static unsigned long shrink_page_list(struct > list_head *page_list, >  #ifdef CONFIG_TRANSPARENT_HUGEPAGE > count_vm_event(THP_SWPOUT_FALLBACK); >  #endif > -                                       if (!add_to_swap(page)) > +                                       if (!add_to_swap(page)) { > +                                               /* > +                                                * Minus tail pages > and reset > +                                                * nr_pages. > +                                                */ > +                                               sc->nr_scanned -= > (nr_pages - 1); > +                                               nr_pages = 1; >                                                 goto activate_locked; > +                                       } >                                 } I think you need to add similar logic for the first add_to_swap() in the original code snippet. >> if (!add_to_swap(page)) { >> if (!PageTransHuge(page)) To reduce code duplication, I suggest to add another label to deal with it. activate_locked_split: if (nr_pages > 1) { sc->nr_scanned -= nr_pages - 1; nr_pages = 1; } activate_locked: And use "goto active_locked_split" if add_to_swap() failed. Best Regards, Huang, Ying